The invention belongs to the field of electrical switches and particularly to a reversing switch for large three phase induction motors.
Bulk materials handling conveyors, such as those used in underground coal mines, are examples of electrically driven equipment which commonly requires reversing facilities. Electrical power equipment used near the working face in coal mines must be "explosion proof" to prevent electrical sparks or arcs from igniting any explosive mixture of methane and air which may be present. Hence any electrical reversing switch intended for use on coal face equipment must be in an explosion proof enclosure. Reversing switch mechanisms previously available have not been easily or inexpensively adaptable for use within explosion proof enclosures.
In certain applications, such as the electric drive and controls for a coal plow operating along the face in a longwall mining system, the motor drive is operated forward and then reverse in a cyclic manner. The motor control equipment includes separate forward and reverse contactors which are operated alternately. That arrangement while effective for such special purposes is objectionably bulky and expensive for many other motor reversing applications such as conveyors which run in one direction to move coal out of the mine but is reversed from time to time to move supplies and personnel into the mine.
Prior to the present invention, no fully suitable reversing switch has been available.